This invention relates to data communication equipment or modems.
Modems are devices which employ digital modulation techniques to transmit binary data over analog band limited communication channels. High-speed modems commonly use linear modulation schemes such as quadrature amplitude modulation (QAM).
In linear modulation systems, binary information is collected in groups of M L bits (M is dimensionality and L is the bits/baud which may be fractional) and the resulting sequence is mapped into a sequence of complex-valued signal points, using some coding scheme. The complex sequence is filtered by a shaping filter to limit its bandwidth, and the real and imaginary components of the filtered signal points are used to amplitude modulate the quadrature components of a sinusoidal carrier of some frequency f.sub.c. If the bit rate is R b/s, then Q=R/L is the baud rate of the linear modulation system. The baud rate represents the minimum bandwidth required to transmit the modem signal without introducing distortion. (The actual bandwidth of the shaping filter may be larger, but it is typically proportional to the baud rate.) The baud rate and the carrier frequency together determine the transmission band.
The bandwidth efficiency of a linear modulation system is measured by L, the number of bits it transmits per baud. For fixed rate R, increasing L reduces the baud rate and thus the required bandwidth. However, increasing L also reduces the noise tolerance of the system. Therefore, for a given channel characteristic, there is an optimum tradeoff between the baud rate and the number of bits transmitted per baud.
On channels with a rectangular or brickwall-like spectrum and white noise, the baud rate must be chosen approximately equal to the channel bandwidth. On the other hand, if the channel spectrum shows gradual attenuation, it may be preferable to choose the baud rate large enough such that portions of the attenuated regions are included in the transmission band. A large baud rate results in increased distortion, however, an equalizer in the receiver can compensate for the distortion and the noise enhancement caused by equalization may be more than offset by the improved noise tolerance obtained with a smaller L.
In most commercial high-speed voiceband modems that are available today, the baud rate and carrier frequency and thus the transmission band is often fixed; e.g., Q=2400 Hz and f.sub.c =1800 Hz. Recently, modems were introduced which offer multiple but manually selectable carrier frequencies. In either case, since channel characteristics show considerable variation between different lines or connections, with such modems it is difficult to achieve the best possible performance on all possible lines.